To Fritz Müller   11 February 1868

My dear Sir,

I am working on Sexual Selection, and am much interested in the Subject. You will remember my asking you about the colours of the Sexes in the lower animals and you gave me some information.¹ I now want especially to know any cases of inequality in the number of the two Sexes in any of the lower animals; Especially of cases in which the females are in excess. I want to see the bearing of such facts on Sexual Selection   Any facts in relation to the males fighting or of the attachment of the male to the female would be very useful to me.

Have you ever observed anything in regard to the musical powers of male insects? In short I should be grateful for any hints whenever you might have the time and inclination to write to me on the Subject. I well know that your kindness is unbounded and I fear it makes me rather unreasonable.

My new Book has sold very well, the whole Edition of 1500 copies being disposed of during the first week.²

Believe me, my dear Sir | to be yours very Sincerely | Charles Darwin

I think you mentioned in one of your letters some Crustaceans in which the sexes were very unequal in number. Was it the extraordinary Crustacean with dimorphic males? I am going to look through your Book “Fur Darwin”.³ I know of course the case of the Gelasimus (which differs much in colour in the two sexes) mentioned in your letter of April 1. 1867, in which you say the male much the most abundant; but I think(?) you have mentioned other cases.—⁴